1. Field of the Invention
The present invention relates to an object information acquiring apparatus and a control method thereof.
2. Description of the Related Art
Object information acquiring apparatuses using X-rays and ultrasound waves have been used in many fields requiring non-destructive inspection, mainly the medical field. In the medical field, physiological information (i.e., functional information) about a biological body is effective for discovery of a disease such as cancer. Therefore, study of imaging of the functional information has recently been conducted. As one example of diagnostic methods using the functional information, photoacoustic tomography (PAT), an optical imaging technique, is proposed. While only morphometric information in a biological body is obtained in an X-ray diagnosis or an ultrasound wave diagnosis, both morphometric information and functional information can be non-invasively obtained in photoacoustic tomography.
In photoacoustic tomography, pulsed light generated from a light source is first emitted into an object. Then, an acoustic wave (typically, an ultrasound wave) is generated from an inner tissue of the object absorbing light propagated and diffused in the object by the photoacoustic effect. This acoustic wave is detected by a probe or the like, whereby the imaging of the information of the inner tissue that is a generation source of the acoustic wave is enabled. Change of the received acoustic wave over time is detected at a plurality of portions surrounding the object, and analytical processing (reconstruction) is mathematically performed for an obtained signal, whereby information associated with an optical characteristic value inside the object can be three-dimensionally visualized. This information can be used as morphometric information inside the object. Furthermore, the functional information including an optical characteristic value distribution such as an absorption coefficient distribution inside the object can be obtained from an initial sound pressure generation distribution generated by emitting light into the object.
For example, near-infrared light can be used as pulsed light emitted into the object. The near-infrared light has a property of being likely to transmit through water that constitutes most of a biological body, and to be absorbed in hemoglobin in blood, and therefore imaging of a blood vessel image using morphometric information is enabled. Furthermore, it is possible to know the content rate of oxygenated hemoglobin to total hemoglobin in the blood, namely, an oxygen saturation by using the absorption coefficient distribution obtained by emitting near-infrared light, and also possible to perform imaging of a biological body function. An oxygen saturation distribution serves as an index distinguishing between benignity and malignity of a tumor, and is therefore expected as an effective means of discovery of a malignant tumor.
The calculation of the oxygen saturation is performed by a comparison operation in which measurement is performed a plurality of times with pulsed light having different wavelengths, and a ratio of absorption coefficients calculated respectively is calculated. This uses a principle that since the optical absorption spectra of deoxygenated hemoglobin and oxygenated hemoglobin are different, the respective content rates are found by performing measurement at different wavelength and comparing the spectra.
In a case of the aforementioned imaging, when the comparison operation calculating the ratios is performed with no change, a blood vessel image portion and a background portion cannot be distinguished. Therefore, as described in Xueding Wang, et al. “Noninvasive imaging of hemoglobin concentration and oxygenation in the rat brain using high-resolution photoacoustic tomography”, Journal of Biomedical Optics 11(2), 024015 (March/April 2006), the blood vessel image portion and the background portion need to be distinguished and only the blood vessel image portion needs to be processed.
Non Patent Literature 1: Xueding Wang, et al. “Noninvasive imaging of hemoglobin concentration and oxygenation in the rat brain using high-resolution photoacoustic tomography” Journal of Biomedical Optics 11(2), 024015 (March/April 2006)